In magnetic resonance facilities, the volume that can be used for magnetic resonance tomography recordings within the tomograph is limited in all three spatial directions due to physical and technical conditions such as e.g. limited magnetic field homogeneity and non-linearity of the gradient field. A usable recording volume or so-called field of view (FoV) is therefore limited to a volume in which the above cited physical properties lie within predefined tolerance ranges, such that a true-to-original representation of the object to be examined is possible using normal magnetic resonance measurement sequences. In particular, this limited field of view is considerably smaller in an x-direction and a y-direction, i.e. perpendicularly relative to a longitudinal axis of a tunnel of the magnetic resonance facility, than the volume that is delimited by the tunnel of the magnetic resonance facility. In the case of conventional magnetic resonance facilities, the tunnel has a diameter in the range of 60-70 cm, whereas the diameter of the normally usable field of view (in which the above cited physical conditions are within the tolerance ranges) is approximately 10 cm smaller, i.e. in the range of 50-60 cm.
In many applications of magnetic resonance facilities, e.g. in the context of an image-based radiotherapy schedule or biopsy or in an MR-PET hybrid system, a true-to-original and spatially accurate representation of the examination object is nonetheless also required in the margin region between the above described normally usable field of view and the inner wall of the tunnel. In the case of MR-PET hybrid systems, e.g. magnetic resonance recordings are used to determine a human attenuation adjustment for subsequent calculation of a positron emission tomography recording (PET recording). Since regions of the examination object, e.g. the arms of a patient, can be arranged in the above described margin region between the normally usable field of view and the inner surface of the tunnel, precise knowledge of the arrangement of the examination object in the margin region is also required for the purpose of determining the attenuation adjustment for the PET recording.
Since the B0 field has inhomogeneities and the gradient field has non-linearities in the margin region, image points in the magnetic resonance recording appear at shifted positions. This spatial inaccuracy of the image points is also referred to as distortion. In the event of significant distortions in these margin regions, it is often impossible subsequently to compensate for the distortion in the magnetic resonance recording, since the distorted regions in the magnetic resonance recording overlap and therefore cannot be corrected subsequently.